Auto-stopping users exempt bg visible users

When automatically stopping a user, due to it being scheduled for
stopping or because we have exceeded maxRunningUsers, we already make
sure to exclude alwaysVisible users (such as the communal profile). But
we should actually also exclude users that are visible on background
displays. In short, we should exclude any user that is
UM.isUserVisible(). So we do so.

Bug: 423011998
Flag: EXEMPT bugfix
Test: UserControllerTest
Change-Id: I2c9754b876d4b8c0c5384e608884b469aa3e26fb